EFFECTS ON UNDERWATER SOUND 



The most direct applications of reports on internal 

 waves are in connection with underwater sound. Numerical 

 evaluation of the effects is known for only one case, 

 wherein sound rays pass through a sharply defined internal 

 wave at steep angles in shallow water. A wide knowledge 

 of the amplitudes, wavelengths, direction of travel, and 

 geographic and seasonal distributions of internal waves is 

 required to evaluate the importance of these effects. 

 Others, such as the release of gas bubbles (which scatter 

 sound) by passage of a large -amplitude internal wave 

 through saturated sea water, may be important. 



Three fundamental factors affect the velocity of 

 sound in sea water — temperature, salinity, and pressure. 

 Anything that introduces time changes or space changes in 

 these properties will also bring in changes in the velocity 

 of sound in the sea. The spectrum of time changes in the 

 velocity of sound at a fixed point is directly proportional to 

 the spectrum of time changes in any conservative property 

 at that point. The latter spectrum can be determined from 

 temperature measurements (provided heating at the 

 boundaries is neglected), which are by far the easiest and 

 most inexpensive that can be made in the ocean. In two 

 cases an estimated 75 to 95 percent of the variance of a 3- 

 day temperature record in shallow water was caused by 

 internal waves. This estimate accounts for variance at 

 frequencies of free internal waves. The range covers 

 periods from about 5 minutes to 24 hours at 30°N. 



Frequency Channels 



Sound velocity in the ocean has a minimum at a 

 depth of about 400 meters. The velocity minimum is the 

 cause of the sound channel that has its axis at the level of 

 the velocity minimum. Most of any omnidirectional sound 

 released near the minimum is retained in the channel by 

 refraction. This is a process by which direction of sound 



